(a)Field of the Invention PA1 (b)Description the Prior Art
The present invention concerns the heat treatment of articles including hardening thereof in a gaseous medium. In this type of technique, the hardening gas is caused to recirculate in contact with said treated articles, the hardening gas being cooled by means of an external heat exchanger.
Heat treatments carried out under vacuum or under a partial pressure of a neutral or cementation gas, in so called vacuum furnaces, often require a rapid cooling of the load at the end of the cycle. Hardening in gaseous medium constitutes an advantageous replacement solution over hardening in liquid medium, because it permits a better control of the kinetics of cooling, it minimizes deformations of the articles treated and takes away the cleaning operations required on the hardened articles, which are carried out in a salt bath or an oil bath.
In practice, furnaces operating under vacuum and adapted for gas hardening are provided with a powerful recirculating blower and the gas injected under pressure within the furnace is successively displaced through the load under the treatment. Cooling by convection therefore takes place while the gas thus warmed up is thereafter sent through a heat exchanger, for example of the water type, which cools the gas before the latter is sent back on the load of articles under treatment.
For a given furnace and load to be treated, the cooling speed of the articles under an atmosphere depends among other factors of the pressure, the recirculation speed and the nature of the gas.
For an existing furnace, the gas speed depends on the recirculating blower. The pressure is controlled by the quantity of gas injected into the furnace; it is limited by the characteristics of the furnace which define a maximum pressure allowable therein.
The gases currently used for hardening are nitrogen and argon. In order to improve cooling speeds without modifying the pressure of the hardening gas or its recirculation speed, it has been proposed to use gases having a better heat conductivity: hydrogen or helium. This promotes heat exchange by convection between the article to be treated and the cooling flow. An improved cooling speed enables to harden a wider range of materials under gas; for a given treatment, it is also possible to decrease the gas pressure in the furnace, thereby lowering the constraints to which the material is subject, or to increase the quantity of material that can be treated during a cycle.
The use of hydrogen which is a flammable gas, requires adequate safety measures on the hardening furnaces. Helium, which is another neutral gas may on the contrary be used instead of nitrogen or argon, without technically modifying the furnace. Because of its high cost, it is still not much in use.